Air Pollution and Free Radical Protection Responses of Plants

Abstract

SOURCES AND COMPOSITIONS OF ATMOSPHERES HARMFUL TO PLANTS A wide variety of primary pollutants are emitted into the atmosphere as a consequence of human activity or from natural sources such as volcanoes and swamps (Wellburn 1994). The major primary emissions are SO 2 , either from the smelting of S-rich ores or the combustion of S-rich fossil fuels in power plants, and nitric oxide (NO), which is created from the nitrogen of the atmosphere by combustion, mainly by transportation systems. NO is then oxidized to NO 2 either by O 3 or by other oxidants in the atmosphere. All atmospheric oxidations are mediated by free radicals. The most important radical in the troposphere is the hydroxyl radical, •OH, which, in turn, controls the cycling and removal of many other air pollutants (e.g., CH 4 ). The production, recycling, and destruction of •OH have been discussed fully by Ehhalt et al. (1991). Meanwhile, NO 2 is photolyzed in the presence of unburnt hydrocarbons to NO and oxygen atoms, which then react with O 2 to form O 3 , the major secondary pollutant of the atmosphere. Uptake of SO 2 , NO, NO 2 , and O 3 into plants, and the relationship between SO 2 , the oxides of nitrogen, and O 3 with phytotoxicity and injury, have been discussed fully elsewhere (see Garsed 1985; Wellburn 1990, 1994, respectively). FORMATION OF VARIOUS FREE RADICALS FROM AIR POLLUTANTS Free radicals have unpaired electrons and can react to acquire or lose an electron to become more stable. Consequently, they may act as oxidants or reductants. Most common air pollutants like...